The aldehyde shown is mixed with ethanal in dilute NaOH. Counting only aldol addition products (beta-hydroxy aldehydes) as different constitutional isomers, and ignoring stereoisomers, how many different products can form? Give the answer as an integer.

The drawn compound is propanal. Both aldehydes have alpha-hydrogens, so each can act as nucleophile and electrophile: ethanal + ethanal, propanal + propanal, ethanal enolate + propanal, propanal enolate + ethanal give four different constitutions (checked with RDKit canonical SMILES). Answer: 4.